This invention relates generally to semiconductor chip device assembly, and in particular to flip chip device assembly. More specifically, the invention relates to a heat sink having an elevated heat spreader lid for a semiconductor package and an assembly process for such a package.
In semiconductor device assembly, a semiconductor chip (also referred to as an integrated circuit (IC) chip or xe2x80x9cdiexe2x80x9d) may be bonded directly to a packaging substrate, without the need for a separate leadframe or for separate I/O connectors (e.g., wire or tape). Such chips are formed with ball-shaped beads or bumps of solder affixed to their I/O bonding pads. During packaging, the chip is xe2x80x9cflippedxe2x80x9d onto its active circuit surface so that the solder balls form electrical connections directly between the chip and conductive traces on a packaging substrate. Semiconductor chips of this type are commonly called xe2x80x9cflip chips.xe2x80x9d
In a conventional method for packaging a semiconductor flip chip a semiconductor die and a packaging substrate are electrically connected and mechanically bonded in a solder joining operation. For example, an unbonded flip chip may have an array of solder balls or bumps arranged on its active circuit surface. The solder is generally composed of a low melting point eutectic material or a high lead material, for example.
The die is aligned with and placed onto a placement site on the packaging substrate such that the die""s solder balls are aligned with electrical traces on the substrate. The substrate is typically composed of a laminate or organic material. Heat is applied to one or more of the die and the packaging substrate, causing the solder balls to reflow and form electrical connections between the die and the packaging substrate. The package is then cooled to harden the connection. An underfill material is also generally applied in order to enhance the mechanical bonding of the die and substrate. In this manner the process produces an electrically and mechanically bonded semiconductor chip assembly, with the underfill material allowing a redistribution of the stress at the connection between the die and the substrate from the solder joints only to the entire sub-die area.
Semiconductor packages are typically subject to temperature cycling during normal operation. In order to improve the thermal performance and reliability of the packages, heat sinks are commonly used. Semiconductor heat sinks typically have stiffener and heat spreader lid components.
A picture frame stiffener is typically placed around the die on the substrate where it is bonded with a heat curable adhesive. The stiffener is typically a flat piece of high modulus metal having substantially the same dimensions as the package substrate with an opening in its center to clear the die. The thickness of the stiffener is typically about the same or slightly more than the height of the die bonded on the substrate. Typically, the stiffener is composed of a copper-based material, e.g., (nickel-plated copper) which has a coefficient of thermal expansion similar to that of typical substrate materials. The purpose of the stiffener is to constrain the substrate in order to prevent its warpage or other movement relative to the die which may be caused by thermal cycling during operation of an electronic device in which the package is installed. Such movement may result from the different coefficients of thermal expansion (CTEs) of the die and substrate materials, and may produce stress in the die or the package as a whole which can result in electrical and mechanical failures.
A heat spreader lid, typically composed of a high thermal conductivity material, and having substantially the same dimensions as the package substrate, is attached over the picture frame stiffener and the die. A conventional heat spreader is typically a flat piece of copper-based material, e.g., (nickel-plated copper). The heat spreader is bonded to the stiffener by a thermally conductive adhesive which is also then heat cured. The heat spreader typically connects with the inactive (non-substrate bound) surface of the die via a thermal grease or adhesive. The purpose of the heat spreader is to disperse the heat generated during thermal cycling in order to reduce stress in the package due to different CTEs of the various elements of the package, including the die, substrate and underfill.
A problem with such flip chip package constructions is that during thermal cycling, the whole package is highly stressed due to the different coefficients of thermal expansion (CTEs) of the substrate and die materials. Shrinkage of the substrate, typically having a CTE of about 17 ppm, is much more than that of the die, which typically has a CTE of about 4 ppm. The high stress experienced by these bonded materials during cooling may cause them to warp delaminate or crack. Such stress in the semiconductor package may ultimately result in its electronic and/or mechanical failure, including cracking of the die, substrate or their solder electrical connections. This problem is particularly acute for larger die sizes, for example dies having dimensions at or in excess of 20 mm on a side, which are presently being fabricated and packaged.
Accordingly, what is needed are apparatuses and methods for improving the reliability of flip-chip packages by constraining warpage during thermal cycling, particularly for large die size packages.
To achieve the foregoing, the present invention provides a heat sink designed to constrain warpage during thermal cycling. The heat sink is composed of a picture frame stiffener and a heat spreader lid. The stiffener has a thickness greater than that of the height of a die bonded on a substrate in the package which the heat sink is or is to be used. The increased thickness of the stiffener beyond conventional designs serves to increase its stiffness and thereby enhance its capacity to constrain the substrate to prevent its warpage during thermal cycling. The thickened stiffener is coupled with an elevated heat spreader lid, that is, a heat spreader lid with a elevated central portion configured for engagement with the picture frame stiffener such that a central portion of at least one side of the heat spreader lid extends into the opening in the picture frame stiffener. The elevated portion of the heat spreader lid extends sufficiently into the central opening in the stiffener to make contact with the inactive (non-substrate bound) side of the die in the package which the heat sink is in or is to be used. Thermal grease is generally used at the joint of the heat spreader lid and the die. As a result, the heat spreader lid is in a position to constrain warpage of the package by applying a counter force against a warping die.
In one aspect, the invention pertains to a semiconductor package heat sink. The heat sink includes a picture frame stiffener having a central opening, and a heat spreader lid configured for engagement with the picture frame stiffener such that a central portion of at least one side of the heat spreader lid extends into the opening in the picture frame stiffener.
In another aspect, the invention pertains to a semiconductor package. The package includes a packaging substrate, a picture frame stiffener having a central opening bonded on a die side of the substrate, a semiconductor die bonded on its active side within the central opening on the die side of the substrate, and a heat spreader lid engaged with the picture frame stiffener such that a central portion of at least one side of the heat spreader lid extends into the opening in the picture frame stiffener and engages the back side of the die.
In another aspect, the invention pertains to a method of packaging a semiconductor die. The method involves bonding the die to a substrate, bonding a picture frame stiffener having a central opening around the die on the substrate, and bonding a heat spreader lid to the stiffener. The heat spreader lid configured such that a central portion of at least one side of the heat spreader lid extends into the opening in the picture frame stiffener and engages the die.